1. Field of the Invention
The invention relates to optical disc drives, and particularly to radio frequency zero crossing signal generators for optical disc drives.
2. Description of the Related Art
FIG. 1 shows essential signals for optical disc tracking, comprising a tracking error signal TE, a tracking error zero crossing signal TEZC, a radio frequency ripple RFRP, and a radio frequency zero crossing signal RFZC. The optical disc drive precedes a cross track motion, and the optical pick-up of the optical disc drive moves from the inner track to the outer track. In such a case, the waveform of the radio frequency ripple is similar to a sinusoid wave. The peaks of the radio frequency ripple RFRP indicate the locations of tracks. The wave troughs of the radio frequency ripple RFRP indicate that the optical pick-up of the optical disc drive is right between the tracks, and no data is obtained. The optical disc drive evaluates an average level for the magnitude of the radio frequency ripple RFRP (ZC), and compares the radio frequency ripple RFRP with the average level ZC to generate the radio frequency zero crossing signal RFZC. As shown in FIG. 1, when the optical disc drive makes a cross track motion, the tracking error signal TE detected by the optical pick-up is also similar to a sinusoid. When the optical pick-up reads a track or is right between the tracks, the tracking error signal TE is zero. The tracking error zero crossing signal TEZC is generated by comparing the tracking error signal TE with zero ‘0’. When the radio frequency zero crossing signal RFZC is high, conventional methods of tracking an optical disc determine the locations where the tracking error zero crossing signal TEZC switches from high to low or from low to high as the locations of tracks. Referring to FIG. 1, the marks 102 indicate the locations of tracks.
In conventional methods of tracking an optical disc, the accuracy of tracking result is dependent on the accuracy of the average level ZC. The upper waveform of FIG. 2 shows a radio frequency signal RF during a cross tracking motion, wherein there are a defect (e.g. a scrape) in region 202 and a fingerprint defect in region 204. The optical pick-up scans the optical disc from the inner track to the outer track and generates the radio frequency signal RF dependent on the reflection of the disc. Because the frequency of the radio frequency signal RF is very high, the waveform of the radio frequency signal RF shown in FIG. 2 is simplified and only the top edge and the bottom edge of the radio frequency signal RF are shown. The lower waveform of FIG. 2 shows a radio frequency ripple RFRP corresponding to the radio frequency signal RF. The RFRP signal is generated by inverting the bottom edge of the radio frequency signal RF. The defect (e.g. scrape) damages the cover coating of the optical disc, so that no radio frequency signal RF can be read out (referring to region 202 of the radio frequency signal RF). Thus, the radio frequency ripple RFRP corresponding to the defect region 202 is high. The average level of the radio frequency ripple RFRP is dramatically shifted upward in the defect region 202. It requires a long time for conventional techniques to restore the dramatically shifted average level to a normal range. Referring to fingerprint defect region 204, the reflection of the optical disc is changed, and the average level of the radio frequency ripple signal RFRP is shifted accordingly. As shown in FIG. 2, the fingerprint defect region 204 dramatically shifts the average level of the radio frequency ripple RFRP upward. Conventional tracking techniques cannot catch up with the dramatic variation of the average level. The average level evaluated by the conventional techniques is usually inaccurate, thus the tracking result is unreliable. Tracking techniques generating real-time and accurate average level of the radio frequency ripple RFRP are thus called for.
FIG. 3 shows a conventional radio frequency zero crossing signal generator 300, comprising a peak hold circuit 302 and a bottom hold circuit 304 implemented by charge pumps. The peak hold circuit 302 and the bottom hold circuit 304 both receive the radio frequency ripple RFRP, and generate the peak value PH and the bottom value BH of the radio frequency ripple RFRP, respectively. The average of the peak value and the bottom value, (PH+BH)/2, is evaluated by an operation circuit 306, and is considered as the average level of the radio frequency ripple RFRP (ZC). The radio frequency zero crossing signal RFZC is generated by a comparator 308 by comparing the radio frequency ripple RFRP with the average level ZC.
Because charge pumps have fast response, the conventional radio frequency zero crossing signal generator 300 is capable of evaluating accurate average level ZC, and the radio frequency zero crossing signal RFZC output from the radio frequency zero crossing signal generator 300 has high accuracy. The drawback of the generator 300 is that the charge pumps are costly.
Cheaper radio frequency zero crossing signal generators capable of providing accurate average level ZC are thus called for.